Magnetic brush development of electrostatic images utilizing a high voltage corona



I Feb. 20, 1968 GRANZQW ET AL I 3,369,917

MAGNETIC BRUSH DEVELOPMENT OF ELECTROSTATIC IMAGES UTILIZING A HIGHVOLTAGE CORONA Filed Sept. 10, 1965 3 Sheets-SheetZ 5 L/OA/A/ L. TEE-GAYArrow/5Y5 DAA/IEL 5. GEAA/ZOIA/AA/D 1 777mm, mam, fizf/zwmmd @u. v

Feb. 20, 1968 GRANZQW ET AL 3,369,917 V MAGNETIC BRUSH DEVELOPMENT OFELECTROSTATIC IMAGES UTILIZING A HIGH VOLTAGE CORONA Filed Sept. 10,1965 -3 Sheets-Sheet 5 MMm'LXMMmMMe m (a MflzM rt, Md W 44 United StatesThis invention relates to an electrostatic copying apparatus, and moreparticularly, to such an apparatus including new and improved means fordeveloping an electrostatic image.

One type of electrostatic printing or copying equipment now in extensiveuse includes a copying sheet having a substrate of paper to one surfaceof which a layer of photoconductive material, such as zinc oxide, in aresin binder is applied. The copying machine includes means for applyinga uniform electrostatic charge to the coating surface of the sheet andthen for selectively illuminating the coated surface in accordance withthe original to be reproduced to provide an electrostatic image orpattern of charged and substantially charge-free areas corresponding tothe original. A developer material such as an electroscopic powder isthen applied to the electrostatic image by such diverse means as amagnetic brush or a cascade to be selectively held on the copy sheet inaccordance with the charged and charge-free areas to form a powder imageloosely held on the sheet. This powder or visible image is placed in apermanent form by heating it in a fuser oven or by contact with a fixingagent.

One of the problems commonly encountered in developing electrostaticimages is the tendency of the developer or electroscopic powder to clingto the substantially charge free-areas during the production of positiveprints. It is believed that this retention of the developer powder inthe background or nonimage area results from a residual charge on theseareas that is not dissipated during the selective illumination or acharge that is developed by the brushing or cascading action of thedeveloper. This adherence of the electroscopic powder or developermaterial, commonly a thermoplastic resin, produces streaks, darkdeposits, or blotches in the white backgorund area of the finished copy.Several different approaches to preventing this retention of thedeveloper powder on the background areas have been tried in the past indifferent types of equipment.

As an example, Walkup et al. Patent No. 2,573,881 discloses anelectrostatic printing apparatus in which layers of dielectric materialare fastened to the outer surface of a highly conductive metal drum inaccordance with the material to be repetitively printed, and the outerperiphery of the drum is then charged with the charge being retainedonly on the insulated layer to provide an electrostatic image. Thisimage is then moved into a cascade developer unit in which a quantity ofdeveloper powder and carrier particles is continuously poured over theouter surface of the metal drum to be electrostatically attracted to andheld by only the charged dielectric layers. To prevent or at leastreduce the undesired retention of the developer powder on the backgroundor nonimage areas of the metal drum, a wide area electrode is spacedoutwardly from the drum so that the electroscopic powder pours downbetween the drum and the developing electrode. This electrode isconnected to a potential source of the same polarity as the image chargeon the drum so as to induce an opposite charge in the exposed portionsof the metal drum. This induced charge is of the same polarity as theelectroscopic powder and tends to repel the powder from the bare metalor background area of the drum.

atcnt Another approach to providing clear background areas in machinesof the type using photoconductively coated sheets of copy paper isillustrated in Sugarman et al. Patent No. 3,058,444. In the apparatusshown in this patent, the electrostatic image on the coated surface ofthe copy sheet is contacted by a magnetic brush comprising a mixture ofiron particles and toner or developer material in the presence of amagnetic field to selectively deposit the electroscopic powder on theimage. To prevent the deposition of powder on the nonimage or backgroundareas, a wide area electrode, either bare or covered by a dielectricsheet, is disposed adjacent the back of the sheet in the area in whichthe coated surface carrying the electrostatic image is engaged by themagnetic brush. This electrode is connected to a potential of the samepolarity as the developer powder to develop a charge, by contact orinduction, on the back of the sheet that is of the same polarity as thecharge on the electroscopic powder. This charge is stated as having theeffect of neutralizing the undesired charge on the background areas onthe other or coated surface of the sheet to prevent the adherence of theelectroscopic powder in these areas.

However, both of these techniques are subject to a number ofdeficiencies in addition to the somewhat unpredictable cleaning of thebackground areas that make them unsatisfactory for use in commercialmachines of the type using photo-conductively coated sheets. Theapparatus shown in the Walkup et al. patent requires a large areaelectrode spaced from the charged surface to induce an opposite polaritycharge in the background area. This electrode must be relatively closeto the charged surface to effectively induce a uniform charge in themeal drum. It must also be spaced away from the drum sufficiently topermit the flow of the electroscopic powder therebetween andsufficiently to prevent the attraction of the electroscopic powder tothe developing electrode. This means that this charge induction methodand apparatus cannot be used with machines in which the electroscopicpowder is applied with a magnetic brush. The other technique suffersfrom the basic fault that a large area electrode must overlie and bedisposed immediately adjacent the developing area to contact the back ofthe copy sheet during developing so as to induce a charge thereon. Thislarge area electrode is contacted by and attracts particles from themagnetic brush during the not inconsiderable periods of time duringwhich a copy sheet is not passing through the developer unit between thebrush and electrode. This deposits iron and electroscopic powder on theelectrode or the dielectric covering sheet therefor which is transferredto the back of the copy sheet subsequently fed into the developer unitwith the result that the backs of the sheet become gritty and streakedwith fused electroscopic powder.

Accordingly, one object of the present invention is to provide a new andimproved apparatus for and method of developing electrostatic images.

Another object is to provide an electrostatic copying machine includingmeans for applying a corona discharge to one side of the copy sheetwhile contacting the opposite side with a magnetic brush containing anelectroscopic powder.

Another object is to provide an electrostatic copying or printingapparatus utilizing a radically new concept of developing electrostaticimages in which a current is forced through a photoconductively coatedcopy sheet simultaneously with the application of an electroscopicpowder.

A further object is to provide a developer for an electrostatic copyingmachine of the type using a magnetic brush in which all structuresengaging a copy sheet or web are spaced from contact with the magneticbrush to avoid 3 smearing copy sheets with developer powder collected onthe structures.

A further object is to provide a developer unit for an electrostaticcopying apparatus including adjustable means for feeding a copy sheetinto engagement with the magnetic brush.

Another object is to provide a method of developing electrostatic imageson photoconductively coated paper sheets with a magnetic brush in whicha current is continuously passed through the sheet and the particles ofthe brush to insure a potential difference betwen the brush and thesheet for controlling the deposition of electrostatic powder on thenonimage or background areas of the sheet.

In accordance with these and many other objects, an embodiment of thepresent invention comprises a developer unit including a rotatablemagnetic brush arranged so that the bristle portions are formed in anupper portion of its cycle of rotation. The developer unit includes adeveloper guide assembly adjustably mounted on the housing and disposedabove the path of movement of the outer extremities of the bristles ofthe magnetic brush. Surfaces on the guide assembly are adapted to engagethe back of a copy sheet supplied from an exposure station or unit withthe electrostatic image facing downwardly. The guide assembly directsthe exposed surface of the sheet against the magnetic brush so that thelatent or electrostatic image is developed by the selective applicationof the developer powder.

To provide means for controlling the deposition of the developer powderin the nonimage or background areas, a corona discharge wire is mountedon the guide assembly overlying the portion of the path of rotarymovement of the magnetic brush in which the magnetic brush contacts thecoated or image bearing surface of the copy sheet. The application ofthe corona discharge to the reverse of the back surface of the copysheet in the area in which it is engaged by the magnetic brush not onlyprevents the adherence of the developer powder to the nonimage areas ofthe sheet when positive printing is being performed but also intensifiesthe image when reversal printing is being performed. The copy sheetguide means and the corona discharge means are spaced from the outerends of the bristles of the magnetic brush so that none of thestructures in the developer unit are engaged by the brush to pick upelectroscopic powder or iron particles that can be subsequentlydeposited on another sheet. Further, even though the developer unit doesnot require any contact between the copy sheets and the developerelectrode in the area in which the coated surface of the sheet isengaged by the magnetic brush, the biasing arrangement made possible bythe corona discharge wire insures perfectly clear background areasduring positive printing and a uniform intensity of the image areasduring reversal printing.

Many other objects and advantages of the present invention will becomeapparent from considering the following detailed description inconjunction with the drawings in which:

FIG. 1 is a series of five schematic diagrams illustrating the stepsperformed in making a copy of an original by an electrostatic process inaccordance with the present invention;

FIG. 2 is a sectional view of a developer unit included in anelectrostatic copying machine;

FIG. 3 is a fragmentary end elevational view of the developer unit;

FIG. 4 is a fragmentary perspective view of a combined paper guide andbias assembly included in the developer unit; and

FIG. 5 is a schematic diagram illustrating the biased developing methodembodying the present invention.

Referring now more specifically to the drawings, therein is illustrateda developer unit (FIGS. 2 and 3) which embodies the present inventionand which is adapted for use in an electrostatic copying or printingmachine. The developer unit 10 develops an electrostatic image on a copysheet 12 (FIG. 2) by the selective application of a developer powder tothe image areas and includes means for preventing the deposition of thedeveloper powder on the nonimage or the background areas. Theconstruction of the unit 10 is such that the deposition of the developermaterial on the back surface of the copy sheet 12 is prevented toprovide a completed copy having not only clear background or nonimageareas but also a clean back surface.

Electrostatic printing machines of the type in which the developer unit10 can be used commonly perform the sequence of operational orfunctional steps illustrated schematically in FIG. 1 to produce a copyof an original on the copy sheet or web 12. The sheet 12 comprises(FIG. 1) a paper layer or substrate 14 having an electrical conductivityon the order of from 10 mhos/ centimeter to 10* mhos/ centimeter. Aphoto-conductive layer 16 is coated on one surface of the substrate 14and can comprise zinc oxide or selenium in a resin binder of thesilicone-alkyd type. The layer of photoconductive coating 16 has aconductivity on the order of from 10- to 10- mhos/ centimeter andexhibits the capacity of accepting a charge of between and 300 volts.

\Vhen the copy sheet 12 is fed into electrostatic printing or copyingmachine, it is fed between a positive corona discharge assembly 18(FIG. 1) and a negative corona discharge means 20 so that positive andnegative charges are applied to the opposite surfaces of thephotoconductive layer 16. In FIG. 1, the positive and negative chargesenclosed in circles represent the charges applied by the assemblies 18and 20, respectively, with the negative charges enclosed withinrectangles representing the net negative charge on the outer surface ofthe photoconductive layer 16.

The charged sheet 12 is then transported to an exposing unit or stationin which light from a source 22 is selectively applied to thephotoconductive layer 16 of the copy sheet 12 under the control of anoriginal 23 to be copied. The selective illumination of the areas orsegments of the photoconductive layer 16 places these areas in aconductive state so that the positive and negative charges applied bythe corona discharge assemblies 18 and 20 are dissipated in theilluminated areas and are retained in the areas that are not exposed toillumination. As illustrated in FIG. 1, it appears that the illuminatedareas of the layer 16 do not become fully discharged and thus retain aresidual or remnant charge. This may be due to the failure of theincident light energy to raise all of the electrons to the conductionband. The exposed copy sheet 12 is then transported to the developerunit 10 with the electrostatic image comprising charged and charge-freeareas disposed on its lower surface or the outer surface of thephotoconductive layer 16.

In the developer station, the unit 10 selectively applies a developer ortoner powder, such as a thermoplastic resin, to the electrostatic imageto provide a powder or a visible image corresponding to the original 23to be copied. In general, the developer unit includes a magnetic brushmeans 24 including a rotating cylinder disposed at least partially in amixture of magnetic material, such as iron filings, and the tonermaterial. During a portion of its path of revolution, the cylinder inthe magnetic brush assembly 24 is subjected to the influence of amagnetic field which forms the iron filings into a bristle-likeconfiguration carrying the toner material. The toner material and theiron filings interact so that the toner particles receive a positivecharge by the triboelectric effect to condition them to be attracted tothe negatively charged areas forming the image.

As the copy sheet 12 is fed into the developer unit 10 withelectrostatic image on the lower surface of the photoconductive layer 16facing downwardly toward the magr nctic brush 24, the bristles of themagnetic brush sweep over the lower surface of the photoconductive layer16 including both the charged and the non-charged areas to permit thepositively charged toner or developer particles to be attracted to andloosely held on the layer 16 by the negative charges carried thereon. Itis believed that this brushing action between the bristles of themagnetic brush means 24 and the photoconductive layer 16 generatesadditional negative charges which are shown in FIG. 1 as enclosed intriangles. The electrical charges resulting from the brushing action ofthe magnetic brush means 24 are deposited on the substantiallycharge-free areas of the background and could combine with any residualnegative charges to cause the loose adherence of the toner or developerparticles, shown as black dots in FIG. 1, to the background areas.

The developer unit 11 includes a developer biasing assembly 2-6 forovercoming the effect of these charges in the background or nonimageareas to prevent the retention of the developer powder. The developerbiasing means 26 includes a corona discharge Wire 28 disposed in ahousing 30 affording a ground plane and connected to a potential source32 that supplies the corona wire 28 with a positive potential on theorder of from 4,000 to 6,000 volts. The corona discharge wire 28 appliesa positive corona discharge to the back or upper surface of the paperlayer 14 in the copy sheet 12 in the area in which the lower surface ofthe photoconductive layer 16- is engaged by the bristles of the magneticbrush means 24. The positive corona discharge provided by the means 26prevents the retention of the toner particles on the background ornonimage areas of the copy sheet 12 without changing the density orclarity of the image areas in which the positively charged particles aredeposited on the copy sheet 12. When the copying machine of which thedeveloper unit forms a part is used for reversal printing, the developerbias assembly 26 has the effect of permitting and of providing a uniformdensity deposition of the toner powder on the noncharged image areas andof preventing the undesired retention of developer iron in the chargednonimage areas. In this application, the toner powder can be of anysuitable type but preferably is of the reversal type disclosed in acopending application of Harmon et al., Ser. No. 221,888, filed Sept. 6,1962, now abandoned.

When the copy sheet 12 is discharged from the developer unit 10, it isforwarded by suitable conveying means to a fusing or fixing station inwhich a heating means 34 applies heat to the thermoplastic resin formingthe toner particles and cause them to fuse to the exposed or outersurface of the photoconductive layer 16 on the copy sheet 12. Thus, theloosely adhered powder image provided in the developer station 10 is nowplaced in permanent form. Although FIG. 1 schematically illustrates theuse of a heating means 34, other fixing means such as a liquidcontacting means can be used to fix the powder image in a permanentform.

Referring now more specifically to FIGS. 24 of the drawings, therein isillustrated the mechanical construction of the developer unit 10embodying the present invention. In general, the unit 10includes asupporting or frame structure having a pair of opposed side or end walls36 on which the remaining components of the unit 10 are mounted. Theunit 10 is supported on the main frame of the electrostatic copyingmachine interposed between the exposing apparatus and the fusing orfixing unit. The magnetic brush assembly 24 and the bias assembly 26 aswell as a copy sheet guiding assembly are mounted on and disposedbetween the spaced sidewalls 36. These components are supported on theend plates 36 so that the magnetic brush assembly 24 is disposedsubstantially below the path of movement of the copy sheet 12 throughthe unit 11 in a position to apply the developer powder to theelectrostatic image on the lower surface of the copy sheet 12, and thedeveloper bias means 26 together with the sheet guiding means aredisposed above the path of movement of this sheet.

The magnetic brush assembly 24 can be of any suitable construction butpreferably comprises a construction of the general type shown anddescribed in detail in Streich, Sr. Patent No. 3,003,462. The magneticbrush means or assembly 24 includes a somewhat trough-like member 38 ofdielectric material disposed between the end walls 36 and defining agroup of communicating troughs or channels in which the mixture ofmagnetic material and toner or developer powder material is disposed.Two of the troughs defined by the member 38 receive feeding anddistributing elements 40 and 42 for uniformly distributing the developermixture along the length of a rotatable cylinder or drum 44 in which apermanet magnet 46 is disposed. The cylinder 44 is rotated in aclockwise direction (FIG. 2) so that bristles of magnetic material mixedwith toner particles formed on its outer periphery in the portionsthereof in which the magnetic field of the permanent magnet 46 iseffective are rotated upwardly to an area in which they are brushedagainst the lower surface of the copy sheet 12. A doctor blade orscraper 48 engages the periphery of the cylinder 44 during a portion ofits path of travel following the point of contact with the copy sheet 12to remove the bristles from the drum.

The exposed sheet 12 is fed into the developer unit 10 from the exposingstation by feeding means so that the leading edge of the copy sheet 12engages a dielectric paper guide element 50 (FIG. 2) disposed at theinlet to the developer unit 10. This leading portion of the copy sheet12 slides over the guide 50 into proximity to the rotating drum orcylinder 44 in the magnetic brush assembly and is held in engagementwith the bristles on the magnetic brush by a guide assembly indicatedgenerally as 52. The sheet 12 is advanced during contact With thebristles of the magnetic brush by the feed means in the developingstation until the leading edge of the sheet 12 is received in the nipbetween a pair of feed rollers 54 and 56. Since the lower surface of thecopy sheet 12 now carries the loosely adhered powder image, the feedrollers 54 and 56 preferably are of the construction disclosed anddescribed in detail in Schulze Patent No. 2,894,744. These feed rollersinclude an outer surface of nap or pile material that does not disturbthe loosely adhered powder held on the lower surface of thephotoconductive layer 16 by the charges thereon. The feed rollers 54 and56 advance the sheet 12 through the developer unit 10 following itsdischarge from the feeding means in the exposing station and serve toadvance this sheet into a transporting assembly which advances the sheetto the heating means 34.

The copy sheet guiding assembly 52 (FIGS. 2-4) is mounted on thesidewalls 36 of the developer unit 10 and includes means for engagingthe back surface of the copy sheet 10 to hold this sheet in engagementwith the bristles of the magnetic brush means 24. However, the guideassembly 52 is supported so that all portions thereof are out of contactwith the magnetic bristles and do not receive toner or developerparticles or iron filings during the period in which a sheet 12 is notpassing through the developer unit 10, which particles or filings couldbe subsequently deposited on the back of the next sheet. This preventssmearing, smudging, and discoloration of the backs of the copy sheets12.

The assembly 52 includes a generally L-shaped bracket or angle iron 58having a lower arm 58a secured by a plurality of fasteners59 to adielectric guide member 60 having an inclined front wall to which agenerally U-shaped paper guide 61 is secured. The lower edges of thearms of the paper guide 61 and the lower edge of the member 60 engagethe back surface of the copy sheet 12 to deflect this sheet intoengagement with the magnetic bristles on the rotaing brush assembly 24.However, these edges are disposed above the bristles to avoid contacttherewith.

To provide means for mounting the paper guide assembly 52 on thedeveloper assembly 10, the opposite outer ends of the arm 58a of theangle iron 58 are provided with downwardly depending portions orflanges. A bracket 62 is secured to one of the depending flanges by ascrew 64 and an additional screw 66 having an outwardly extending studportion. A similar screw 68 having a stud portion is secured to thedepending flange at the other end of the arm 58a. These two studs arereceived Within aligned notches 70 (FIG. 3) in the end walls 36 to provide means for pivotally mounting the paper guide assembly 52 on thesewalls. The bracket 62 includes an upwardly extending arm portion 62ahaving an outwardly extending flange to which a nut 72 is secured. Anadjusting screw 74 is received within the nut 72 and includes a lowerend bearing on an upper edge of the wall 36 (FIG. 3). A tension spring76 connected at one end to the flanged arm 62a and at its other end to abracket 73 secured to the side wall 36 by a screw 80 resiliently biasesthe guide assembly 52 for pivotal movement about the studs 66 and 68 ina clockwise direction so that the lower end of the screw 74 bearsagainst the upper edge of the side wall 36.

By adjusting the position of the screw 74, the position of the paperguide elements 60 and 61 in the assembly 52 relative to the periphery ofthe drum or cylinder 44 is adjusted, and the path of movement of thecopy sheet 12 can be controlled. In this manner, the point and degree ofcontact of the lower surface of the copy sheet 12 with the bristles onthe magnetic brush assembly can be controlled and, similarly, thelocation of the paper engaging surfaces of the parts in the assembly 52relative to the bristles can be controlled so that they are notcontaminated with iron filings and toner particles during the intervalsin which a copy sheet 12 is not passing through the developer unit 10.

The bias assembly 26 for applying a positive corona discharge to theback of the sheet 12 in the area in which the front or lower surface ofthe sheet is engaged by the bristles of the magnetic brush assembly 24is carried on the adjustable paper guide assembly 52. More specifically,the paper guide assembly 26 includes a pair of dielectric or ceramicstand-ofl insulators 82 secured in spaced positions to two parallel legsof two angle iron elements 84 and 86 by two machine screws 88. Thecorona discharge wire 28 is secured to the lower depending ends of theinsulators 82 by a pair of dielectric screws 92. One end of the coronadischarge wire 28 is connected to the positive potential supply 32through suitable control means, such as a variable resistor 94 (FIGS. 1and The other two legs 84a and 86a of the angle irons 84 and 86 arespaced from each other to define a slot in which the upper end of theother leg 58b of the angle iron 58 is slidably received so that the biasassembly 26 is mounted on the paper guide assembly 52 with the coronadischarge wire 28 disposed above the area in which the copy sheet 12 isengaged by the bristles of the magnetic brush 24. The angle irons 84,86, and 58 are connected to ground through the frame of the machine toestablish a ground plane for the corona discharge element 90. Since thedeveloper bias assembly 26 is mounted on the paper guide assembly 52,the adjustment of the position of the assembly 52 relative to the drum44 in the magnetic brush assembly 24 concurrently adjusts the spacing ofthe corona discharge wire 28 from the back of the sheet 12 Withoutvarying the spacing between the sheet engaging portions of the assembly52 and the corona element 28.

To provide means for removing any iron filings that inadvertently mayhave become deposited on the back of the sheet 12 during its passagethrough the developer unit 10, a bracket 95 (FIG. 2) is secured betweenthe end walls 36 and carries a housing 96 having a lower edge 96::adapted to engage the entire transverse dimension of the back surface ofthe copy sheet 12 immediately before it passes into the nip of the feedrollers 54 and 56. A permanent magnet 08 disposed Within the housing 96continuously applies a magnetic field to the elements and 96 to removeany magnetic particles that are inadvertently deposited on the backsurface of the copy sheet 12 before it is discharged from the developerunit 10 and conveyed to the fuser unit. Thus, the magnetic cleaningafforded by the permanent magnet 98 positively assures a uniform cleancondition of the backs of the exposed and developed copy sheets 12.

The application of the positive polarized corona discharge to the backof the copy sheet 12 in the area in which the photoconductive layer 16is engaged by the bristles of the magnetic brush means 24 has the resultof preventing the retention of the developer or electroscopic powder onthe non-images areas of the sheet 12, but the bias assembly 26 is ofsuch a construction that it permits the area above the magnetic brushmeans 24 to remain entirely open to prevent the collection andsubsequent deposition of iron filings and electroscopic powder on theback surface of the sheet 12. One possible explanation for the greatlyimproved results obtained with the bias assembly 26 in the developerunit 10 is set forth below with reference to FIG. 5 of the drawings.FIG. 5 is a schematic diagram of electrical circuit including the coronadischarge element 28, the copy sheet 12, and the drum 44in the magneticbrush assembly 24.

When the electrostatic printing machine is adjusted incident to beingplaced in operation, the potentiometer or variable resistor 94 isadjusted to supply a sufficiently high potential on the order of from4,000 to 6,000 volts poled positively with respect to ground to thecorona element 28. By virtue of the ground plane established by thegrounded supports for the corona wire 28, ionization takes place, and aresistance element 100 represents the resistance of the ionizedatmosphere between the corona wire 28 and the back surface of the copysheet 12. An additional resistance element 102 represents the resistanceof the paper substrate 14 of the copy sheet 12, and a resistance element104 represents the resistance of the photoconductive layer 16. Aresistance element 106 represents the resistance of the magneticfiilings bridged between the grounded and highly conductive drum 44 andthe engaged lower surface of the photoconductive layer 16. It has beenexperimentally determined that a minute current on the order ofmicroamperes flows from the grounded drum 44 to the corona dischargewire 28 over the path including the schematically illustrated resistanceelements 100, 102, 104, and 106.

The continuous flow of the small amount of direct current between thecorona discharge Wire 28 and the drum 44 through the bristles of themagnetic brush produces a variety of effects. In the first place, themagnitude of the flow of current is rather small because of the highimpedance of the circuit between the corona wire 28 and the drum 44.However, this current is great enough to carry off the spurious chargesin the nonimage or background areas so that the electroscopic powder isnot attracted to and held in the background areas. Quite surprisingly,the flow of current which also passes through the charged image areasdoes not dissipate this charge sufliciently to have any effect on theclarity or density of the image areas. This is probably due to therelative quantities of the charges on the image and background areas. Inaddition, the flow of current through the developer mix including boththe iron filings and the electroscopic powder insures that the surfaceof the photoconductive layer 16 is more positive than the electricallyconductive iron filings in the developer mix, and this appears to reducethe tendency of the electroscopic material to deposit on the backgroundor nonimage areas of the copy sheet 12. Further, even though the ironparticles are positively biased by the current llow, the electroscopicmaterial on the biased particles become more positive by thetriboelectric effect. This means that the more positively charged tonerparticles are always transferred to the image areas prior to themagnetic filings and will satisfy the charge demand before any iron isdeposited. This phenomena is assisted by the force of magneticattraction acting on the iron particles which must be overcome prior todeposition of the iron in the image areas.

Although the present invention has been described with reference to asingle illustrative embodiment thereof, it should be understood thatnumerous other modifications and embodiments can be devised by thoseskilled in the art that will fall within the spirit and scope of theprinciples of this invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A method of developing the image bearing area on one surface of acopy sheet containing an electrostatic charge without developingnonimage areas on the same surface of the sheet having no more than asmall amount of undesired charge, which method comprises the steps ofapplying a magnetic brush formed by a mixture of magnetic particles anddeveloper powder to the image and nonimage areas on the one surface ofthe copy sheet to permit the retention of the developer powder by thecharge in the image areas, and concurrently applying a voltage in therange from four thousand to six thousand volts to electrode means toapply a corona discharge to the opposite surface of the copy sheet toproduce a direct current flow through the copy sheet of such proportionsas to carry off the undesired charge in the nonimage areas withoutalfecting the ability of the charge in the image areas to retain theapplied developer powder.

2. A method of developing the image bearing area on one surface of acopy sheet containing an electrostatic charge without developingnonimage areas on the same surface of the sheet having no more than asmall amount of undesired charge by the use of a magnetic brush, whichmethod comprises the steps of applying a developer powder to the imageand nonimage areas on the one surface of the copy sheet through themagnetic brush to permit the retention of the developer powder by thecharge in the image areas, connecting the magnetic brush to a referencepotential, applying a voltage in the range of four thousand to sixthousand volts to electrode means to apply a corona discharge to theother surface of the copy sheet in the area oppoiste to that engaged bythe magnetic brush of suflicient magnitude to produce a direct currentflow through the copy sheet of such proportions as to carry off theundesired charge in the nonimage areas without affecting the ability ofthe charge in the image areas to retain the applied developer powder.

3. A method of making a copy of an original on a paper sheet coated on afront surface with a photoconductive material, which comprises the stepsof producing a pattern of charged and substantially charge-free areas onthe coated surface of the sheet in accordance with the original to becopied, applying a magnetic brush formed of a mixture of magneticpatricles and developer powder to successive segments of the coatedsurface embracing both charged and charge-free areas during relativemovement between the magnetic brush and the paper sheet so that thedeveloper powder is selectively retained on the coated surface inaccordance with the charged and chargefree areas, establishing a coronadischarge by connecting a potential in the range of from four thousandto six thousand volts to an electrode means, and applying the coronadischarge to the other surface of the sheet in the area in which thefront surface of the sheet is contacted by the magnetic brush to producea direct current flow through the paper sheet in the segments thereofsuccessively contacted by the magnetic brush of a great enough magnitudeto remove spurious charges from the substantially charge-free areaswithout effecting any substantial change in the charged areas.

4. A method of making a copy of an original on a paper sheet coated witha photoconductive material, which comprises the steps of producing apattern of charged and substantially charge-free areas on the coatedsurface of the sheet in accordance with the original to be copied,moving the sheet along a path with the coated pattern bearing surface ofthe sheet facing downwardly, applying a magnetic brush formed of amixture of magnetic particles and developer powder to successivesegments of the coated surface embracing both charged and charge-freeareas during relative movement between the magnetic brush and the papersheet so that the developer powder is selectively retained on the coatedsurface in accordance with the charged and charge-free areas,establishing a corona discharge by connecting a potential in the rangefrom four thousand to six thousand volts to an electrode means, andapplying the corona discharge to the upper surface of the sheet in thearea in which the downwardly facing surface is engaged by the magneticbrush to produce a direct current flow through the paper sheet in thesegments thereof successively contacted by the magnetic brush of a greatenough magnitude to remove spurious charges from the substantiallycharge-free areas without effecting any substantial change in thecharged areas.

5. An apparatus for developing an electrostatic image on a copy sheetcomprising a magnetic brush assembly for applying a developer powder tothe electrostatic image, guide means spaced from the magnetic brushassembly a sufficient distance to avoid contact with the developerpowder for guiding movement of the copy sheet over the magnetic brushassembly with the electrostatic image facing downwardly to be engaged bythe magnetic brush so that the developer powder is selectively retainedon the surface of the copy sheet in a pattern determined by theelectrostatic image, a corona discharge wire extending transverse to thepath of movement of the copy sheet and disposed above the back of thecopy sheet in substantial alignment with the area of contact of themagnetic brush with the image bearing surface of the copy sheet, and apotential source connected to the corona discharge wire and adapted tosupply a voltage to the wire in the range from four thousand to sixthousand volts.

6. An apparatus for developing an electrostatic image on a copy sheetcomprising a magnetic brush assembly for applying a developer powder tothe electrostatic image, guide means for guiding the copy sheet into aposition with the image bearing surface of the sheet engaging themagnetic brush, said guide means including a transverse- 1y extendingdielectric member having a sheet engaging portion disposed generally toone side of the magnetic brush and spaced from the magnetic brush adistance sufiicient to prevent contact of the member by the brush, saidcopy sheet being fed from said one side into engagement with thedielectric member and said member directing the copy sheet toward themagnetic brush, corona discharge means spaced from the magnetic brushassembly for applying a corona discharge to the back of the copy sheetin proximity to the area of contact between the magnetic brush and theimage bearing surface of the copy sheet, and a potential sourceconnected to the corona discharge means and adapted to supply a voltageto the corona discharge means in the range from four thousand to sixthousand volts, the dielectric member being mounted generally betweenthe magnetic brush and the corona discharge means so that the back ofthe copy sheet is shielded from the corona discharge by the dielectricmember until the corresponding image bearing surface of the copy sheetis contacted by the magnetic brush.

7. The apparatus set forth in claim 6 including means for adjusting theposition of the guide means and the corona discharge means relative tothe magnetic brush assembly.

8. A developer unit for use with a copy sheet having an electrostaticimage on one surface thereof comprising a support, a magnetic brushassembly carried on the support and including means providing a movingbristle-like formation, first and second sheet engaging means mounted onthe support and disposed on opposite sides of the bristle-like formationalong a path of sheet movement through the developer unit, thebristle-like formation being disposed on one side of the path to contactthe image bearing surface of the sheet and the sheet engaging meansbeing disposed on the opposite side of the path to engage the back ofthe sheet, the first and second sheet engaging means being spaced fromcontact with the bristle-like formation to avoid contact with particlestherefrom and the area between the first and second sheet engaging meanson said opposite side of the path being open and free of any structurethat could be contacted by the bristlelike formation, the first sheetengaging means having a guiding suface for engaging the back of thesheet as it is fed into the developer unit along the path and for movingthe image bearing surface into engagement with the bristle-likeformation, a corona discharge unit disposed on said opposite side of thepath and in the open area between the first and second sheet engagingmeans for applying a corona discharge to the back of the sheet in thearea of contact between the bristle-like formation and the image bearingsurface of the sheet, a potential source connected to the coronadischarge unit and adapted to supply voltage to the corona dischargeunit in the range from four thousand to six thousand volts, and astructure on the support located on said opposite side of the path andinterposed between the corona discharge unit and the back of the sheetto prevent the application of the corona discharge to the back of thesheet before the corresponding area of the image bearing surface of thesheet is contacted by the bristle-like formation.

9. The unit set forth in claim 8 in which the structure forms a part ofthe first sheet engaging means.

10. The unit set forth in claim 9 including means mounting the coronadischarge means on the first sheet engaging means.

11. The unit set forth in claim 10 including means for movably mountingthe first sheet engaging means on the support to permit the position ofthe first sheet engaging means and the corona discharge means to beconcurrently adjusted relative to the bristle-like formation withoutchanging their positions relative to each other.

12. In an apparatus for making electrostatic reproductions of a graphicoriginal on an electrophotographic member having an insulatingphotoconductive layer on a paper substrate, the photoconductive layerhaving a latent electrostatic image of one polarity, the combinationcomprising developer means having a magnetic brush made up of magneticparticles and electroscopic powder including magnetic means to form saidmagnetic brush for applying said electroscopic powder to the latentimage, said magnetic brush being connected to ground, a corona chargingmeans for applying a corona discharge of a second polarity opposite tothe one polarity to the back of said member opposite the magnetic brushwhereby said member is driven to a potential removed from groundpotential by conduction, and a potential source connected to the secondcorona charging means and adapted to supply a voltage to the secondcorona charging means in the range from four thousand to six thousandvolts.

References Cited UNITED STATES PATENTS 2,815,449 12/1957 Giaimo117--17.5 X 2,854,947 10/1958 Giaimo 1.7 X 2,890,968 6/1959 Giaimo11717.5 2,904,000 9/1959 Fisher et al 95-17 X 2,951,443 9/1960 Byrne11717.5 X 2,952,241 9/1960 Clark et al 11717.5 X 2,956,487 10/1960Giaimo 951.7 2,965,481 12/1960 Gundlach 117-17.5 X 3,003,462 10/1961Streich 11717.5 X 3,037,478 6/1962 Lace 11717.5 X 3,058,444 10/1962Sugarman et al. 117--17.5 X 3,088,386 5/1963 Sugarman 95-1.7 3,117,8841/1964 Greig 117-17.5 3,145,122 8/1964 Streich 118-637 3,150,003 9/1964Murray et al 11717.5 X 3,232,190 2/1966 Willmott 11717.5 X 2,297,69110/1942 Carlson 11717.5 X 2,965,573 12/1960 Gundlach 11717.5 X 3,011,47312/1961 Gundlach 11717.5 X 3,094,429 6/1963 Howell 11717.5

WILLIAM D, MARTIN, Primary Examiner.

EDWARD J. CABIC, Assistant Examiner.

1. A METHOD OF DEVELOPING THE IMAGE BEARING AREA ON ONE SURFACE OF ACOPY SHEET CONTAINING AN ELECTROSTATIC CHARGE WITHOUT DEVELOPINGNONIMAGE AREAS ON THE SAME SURFACE OF THE SHEET HAVING NO MORE THAN ASMALL AMOUNT OF UNDERSIRED CHARGE, WHICH METHOD COMPRISES THE STEPS OFAPPLYING A MAGNETIC BRUSH FORMED BY A MIXTURE OF MAGNETIC PARTICLES ANDDEVELOPER POWDER TO THE IMAGE AND NONIMAGE AREAS ON THE ONE SURFACE OFTHE COPY SHEET TO PERMIT THE RETENTION OF THE DEVELOPER POWDER BY THECHARGE IN THE IMAGE AREAS, AND CONCURRENTLY APPLYING A VOLTAGE IN THERANGE FROMFOUR THOUSAND TO SIX THOUSAND VOLTS TO ELECTRODE MEANS TOAPPLY A CORONA DISCHARGE TO THE OPPOSITE SURFACE OF THE COPY SHEET TOPRODUCE A DIRECT CURRENT FLOW THROUGH THE COPY SHEET OF SUCH PROPORTIONSAS TO CARRY OFF THE UNDESIRED CHARGE IN THE NONIMAGE AREAS WITHOUTAFFECTING THE ABILITY OF THE CHARGE IN THE IMAGE AREAS TO RETAIN THEAPPLIED DEVELOPER POWDER.